The present invention relates generally to tooling for the assembly of electronic devices and more particularly to a fixture for supporting a printed circuit board and a method of fabricating and utilizing the fixture during the mounting of components on such a board.
Printed circuit boards are used to support and interconnect components such as integrated circuits ("ICs"), resistors, capacitors and the like. There has been a trend for ICs to grow more complex, for the physical sizes of ICs and of other components to shrink, and for the density of components to increase. In order to adequately support and interconnect these very small, highly complex components on printed circuit boards, surface mount printed circuit board techniques have been developed and are coming into widespread use.
Installing components on a printed circuit board by means of surface mount techniques differs from conventional mounting in that the leads of the components are not inserted into holes in the board. Instead, the leads are soldered to solder pads on the surface of the board (the solder pads consist of exposed areas of the printed wiring on the board).
More particularly, surface mounting includes applying a solder paste to all the solder pads on the board, for example by a stencil screen process; placing all the components on the board with their leads in proper alignment with the pads; and applying heat to the resulting assembly, for example by baking it in an oven, to reflow (melt) the solder paste and thereby fix the leads of the components to their respective solder pads.
In order to fit as many components as possible onto a circuit board and thereby minimize the number of boards which must be used to manufacture a complete electronic unit, many circuit boards are now being designed for double-sided component mounting. To install components on such a board requires carrying out the above mounting sequence twice, once to install components on one side of the board and again to install the remaining components on the other side of the board.
The surface of the board must remain flat, and the board itself must be held immobile, during the application of the solder paste and the positioning of the components. The materials of which circuit boards are made are not inherently rigid enough for this purpose, and hence the board must be supported beneath substantially its entire area during component installation. This is easily accomplished during the installation of components on the first side of the board simply by resting the board on a flat work surface such as a rigid metal plate. However, this method does not work when installing components on the second side of the board because the components on the first side protrude and get in the way.
A suitable rigid support can be made by machining a set of depressions into a block of metal, one depression for each component, the depressions being laid out in a pattern which corresponds with the pattern of the components on the first side of the board. In general the components are not all of the same size and hence each depression must be machined to a different size according to the particular component which that depression is to accommodate. In addition, small variations in component placement from one board to another during a production run can result in a misfit between a board and the supporting block.
To assemble a board by means of such a machined support block, the components are installed on the first side of the board as described above; then the board is inverted and placed on the machined support with the components resting in their corresponding depressions; then the solder paste is applied to the second side of the board, the remaining components are positioned, and the assembly is heated to fix the component leads to their solder pads on the second side of the board.
Each different circuit layout requires a different supporting block. Moreover, any change in the physical positioning of components on a circuit board requires that the block be re-machined or in some instances that a new block be prepared. A new block may even be needed just to accommodate a change in the dimensions of a single component. Machining and re-machining these support blocks is an expensive and time-consuming process (the assignee of the present application finds that the cost of fabricating such a block averages nearly $1500) and has been a serious problem in the assembly of double-sided printed circuit boards. The problem is especially acute with respect to the assembly of prototype circuit boards and relatively small production runs.
It will be apparent from the foregoing that there is a need for a simple, economical means of supporting a printed circuit board during the installation of components thereon.